1. Field of the Invention
The present invention relates to an axial air flow fan suitably used for a water cooling system for a water cooled internal combustion engine.
2. Description of Related Art
High grade automobiles are now much in demand. Such vehicles usually employ an internal combustion engine having a larger cylinder volume and high output power, and in this case, a quality cooling system having an increased cooling ability is required. Therefore, the engine must have a cooling fan capable of increased performance. In order to obtain increased cooling performance, a high rotational fan speed is used and the fan must be operated for a longer time, which reduces fuel consumption efficiency of the engine and increases engine noise. In contrast to this, a reduction in a fuel consumption is required of many National or Regional governments such as CAFE and a demand for reduced noise is also required from the viewpoint of the environment. Thus, there is a strong demand to provide a fan with an improved construction capable of reducing noise without reducing the cooling performance.
Japanese Examined Patent Publication No 63-13040 discloses a cooling fan housed in a shroud and provided with a blade having an end portion defining a plane having an increased pitch (mounting) angle with respect to that of the remaining portion of the blade, which is effective for decreasing the strength of a vortex of air generated at the blade end that faces the inner surface of the shroud, with a short distance therebetween. Such a reduction in the strength of the vortex is aimed at reducing the operational noise as well as increasing the cooling efficiency.
However, the inventor's test reveals that, in addition to a vortex at the blade end, a stagnant area having a very low speed is created at the root location of a blade, which also reduces the performance of the fan.